Polymer network liquid crystal element with predetermined gap for dimming device having image processing, temperature detecting and pulse width control units therefor

ABSTRACT

To provide a light control device including a liquid crystal element which does not need a polarizing plate and an alignment layer and is compact and high in contrast ratio, and can be driven at low applied voltage and can exhibit stable performance even if environmental temperature varies, and a driving method for the light control device, as well as an image pickup apparatus using the light control device. A liquid crystal cell ( 1 ) is fabricated by injecting a mixture in which liquid crystal, a polymer precursor, and a polymerization initiator into an empty cell constituted by two transparent substrates ( 8 ) which are stuck together with a gap of 4-11 μm and on each of whose opposed surfaces a transparent electrode ( 9 ) is formed, and the polymer precursor is polymerized and then a random three-dimensional network polymer ( 3 ) is formed in the continuous layer of liquid crystal ( 2 ). The liquid crystal cell ( 1 ) is driven by applying a voltage according to the environmental temperature. When driving voltage is off, the liquid crystal molecules ( 2 ) are aligned along the wall surfaces of the polymer ( 3 ), the network polymer ( 3 ) forms light scattering interfaces, on the other hand, when driving voltage is on, the liquid crystal molecules are oriented in a predetermined direction with respect to the electric field, so that the refractive index becomes constant in the traveling direction of light, whereby incident light ( 5 ) passes through without being scattered.

TECHNICAL FIELD

The present invention relates to a light control device using, forexample, a liquid crystal optical element for adjusting the light amountof incident light and allowing adjusted light to radiate through, and adriving method for the light control device, as well as an image pickupapparatus using this light control device.

BACKGROUND ART

Light control devices using liquid crystal optical elements (liquidcrystal cells) generally use polarizing plates. As these liquid crystalcells, for example, TN (Twisted Nematic) liquid crystal cells andguest-host (GH (Guest Host)) liquid crystal cells are used.

FIGS. 8A and 8B are schematic views showing the operating principle of arelated art light control device.

This light control device is mainly made of a polarizing plate 11 and aGH cell 12 a of positive liquid crystal. The GH cell 12 a is sealedbetween two glass substrates which are not shown, and has operatingelectrodes and liquid crystal alignment films none of which are shown.Liquid crystal molecules 13 a and dichroic dye molecules 14 are sealedin the GH cell 12 a (Δε>0).

The liquid crystal molecules 13 a which serve as a host material are ofthe positive type which have positive dielectric constant anisotropy.The dichroic dye molecules 14 which serve as a host material hasanisotropy in absorption of light, and may be of the positive type (ptype) or the negative type (n type). FIGS. 8A and 8B show an example ofpositive (p type) pigment molecules which absorb light in the directionof their long molecular axis (ΔA>0).

On the other hand, FIG. 8A shows the state of the GH cell 12 a to whichvoltage is not applied.

Incident light 5 is changed into linearly polarized light by beingfiltered while passing through the polarizing plate 11. The polarizationdirection of the polarized light coincides with the direction of thelong molecular axis of the dichroic dye molecules 14, so that thepolarized light is easily absorbed by the dichroic dye molecules 14.Accordingly, the light transmittance of the GH cell 12 a is low duringthe state of FIG. 8A in which voltage is not applied.

FIG. 8B shows the state of the GH cell 12 a to which voltage is applied.When voltage is applied to the GH cell 12 a, the liquid crystalmolecules 13 a are aligned in the direction of the electric field, sothat the direction of the long molecular axis of the dichroic dyemolecules 14 becomes orthogonal to the polarization direction of light.Accordingly, polarized light is hardly absorbed by the dichroic dyemolecules 14 and is allowed to pass through. Accordingly, the lighttransmittance of the GH cell 12 a is high during the state of FIG. 8B inwhich voltage is applied.

Incidentally, negative (n-type) pigment molecules which absorb light inthe direction of their short molecular axis may also be used as thedichroic dye molecules. In this case, the light transmittance isopposite to that in the case where positive pigment molecules are used.Light is not easily absorbed during the non-application of voltage, butduring the application of voltage, light is easily absorbed.

In the light control device shown in FIGS. 8A and 8B, the ratio ofabsorbance during the application of voltage to that during thenon-application of voltage, i.e., the ratio of optical densities, isapproximately 10. This light control device has an approximatelytwo-fold optical density ratio compared to a light control device whichdoes not use the polarizing plate 11 and is made of only a GH cell 12 b.

FIG. 9 is a graph in which the light transmittance of the GH cell 12 ashown in FIGS. 8A and 8B to which driving pulses of rectangular wavesare applied is plotted against driving pulse voltage. An average visiblelight transmittance (a value in the air: the transmittance obtained whenan empty liquid crystal cell and a polarizing plate are placed in anoptical path is defined as a reference transmittance (=100%), and thisdefinition applies to the following description as well) increases withthe increase of the driving pulse voltage, but the maximum lighttransmittance obtained when the driving pulse voltage is increased to 10V is as low as approximately 60% and the variation of the lighttransmittance is modest.

A cause of this is considered to be that since positive liquid crystalmolecules exhibit strong interactions with the interface of a liquidcrystal cell with a liquid crystal alignment film during thenon-application of voltage, a comparatively large number of liquidcrystal molecules whose director, even if a voltage is applied, does notat all vary or does not easily vary in its direction are contained inthe positive liquid crystal molecules.

The present applicant has conducted intensive research and proposed alight control device using a negative liquid crystal as its hostmaterial, as well as an image pickup apparatus using this light controldevice (refer to Patent Document 1. This invention which relates toPatent Document 1 is hereinafter referred to as the invention of thefirst prior application.)

FIGS. 10A to 10C are schematic views showing the operating principle ofa light control device based on the invention of the prior application.This light control device is mainly made of the polarizing plate 11 anda GH cell 12 b similarly to the related art light control device ofFIGS. 8A and 8B. Liquid crystal molecules 13 b (Δε<0) of the negativetype which have negative dielectric constant anisotropy and serve as ahost material and dichroic dye molecules 14 (ΔA=A//−A⊥>0) of thepositive or negative type which serve as a guest material are sealed inthe GH cell 12 b. FIGS. 10A and 10B show the case in which the dichroicdye molecules 14 are pigment molecules of the positive type (p type).

FIG. 10A shows the state of the GH cell 12 b to which voltage is notapplied.

The incident light 5 is changed into linearly polarized light by beingfiltered while passing through the polarizing plate 11. The polarizationdirection of the polarized light is orthogonal to the direction of thelong molecular axis of the dichroic dye molecules 14, so that thepolarized light is hardly absorbed by the dichroic dye molecules 14 andis allowed to pass through. Accordingly, the light transmittance of theGH cell 12 b is high during the state of FIG. 10A in which voltage isnot applied.

On the other hand, FIG. 10B shows the state of the GH cell 12 b to whichvoltage is applied. When voltage is applied to the GH cell 12 b, theliquid crystal molecules 13 b are aligned to become orthogonal to thedirection of the electric field, so that the direction of the longmolecular axis of the dichroic dye molecules 14 coincides with thepolarization direction of light. Accordingly, polarized light is easilyabsorbed by the dichroic dye molecules 14. Accordingly, the lighttransmittance of the GH cell 12 b is low during the state of FIG. 10B inwhich voltage is applied.

Incidentally, negative (n-type) pigment molecules may also be used asthe dichroic dye molecules. In this case, the light transmittance isopposite to that in the case where positive pigment molecules are used.

FIG. 11 is a graph in which the light transmittance of the GH cell 12 bof FIGS. 10A and 10B to which the driving pulses of rectangular wavesshown in FIG. 10C are applied is plotted against driving pulse voltage.At this time, as one example of the negative liquid crystal 13 b havingnegative dielectric constant anisotropy (Δε), MLC-6608 manufactured byMerck KGaA is used as a host material, while D5 manufactured by BDHChemical Co. Ltd. is used as one example of the dichroic dye molecules14 having positive light absorption anisotropy (ΔA). As shown in FIG.11, an average visible light transmittance decreases to several % from amaximum light transmittance of approximately 75% with the increase ofthe driving pulse voltage, and the variation of the light transmittanceis comparatively sharp.

A cause of this is considered to be that since negative liquid crystalmolecules exhibit very weak interactions with the interface of a liquidcrystal cell with a liquid crystal alignment film during thenon-application of voltage, light is allowed to pass through during thenon-application of voltage, and the director of the liquid crystalmolecules is easily varied in its direction by the application ofvoltage.

Accordingly, according to the invention of the first prior application,by constructing a guest-host liquid crystal cell by using a negativeliquid crystal as a host material, it is possible to realize a compactlight control device which is improved in light transmittance during itstransparent state in particular and which enables a GH cell to be fixedin position in an image pickup optical system.

As described previously, in light control devices using GH cells, it ispossible to realize an approximately two-fold optical density ratio (theratio of absorbance during the application of voltage to that during thenon-application of voltage) by using polarizing plates, compared to thecases in which polarizing plates are not used. However, if a polarizingplate is used, at least half light is lost down to a light transmittanceof, for example, 40-50%, so that a remarkable decrease in light amountoccurs. Accordingly, if the polarizing plate is constantly placed in theoptical path of a light control device, there is the problem that themaximum transmittance of the light control device is restricted by thetransmittance of the polarizing plate and sufficient light amountscannot be ensured in dark places.

The present applicant has therefore proposed a light control devicewhich is improved in contrast ratio and can correctly perform lightcontrol operation over a wide range from bright places to dark places bybeing constructed of a liquid crystal element and a polarizing platedisposed for movement into and out of an effective optical path of lightentering this liquid crystal element (refer to Patent Document 2. Thisinvention which relates to Patent Document 2 is hereinafter referred toas the invention of the second prior application.)

The light control device based on the invention of the second priorapplication is disposed between a front lens group 15 and a rear lensgroup 16 each constructed of a plurality of lenses like a zoom lens asshown in FIG. 12 by way of example. Light passing through the front lensgroup 15 enters the GH cell 12 b after having been changed into linearlypolarized light by the polarizing plate 11. The light passing throughthe GH cell 12 b is converged by the rear lens group 16 and formed on animage pickup plane 17 as an image.

The polarizing plate 11 which constitutes the light control device canbe moved into and out of an effective optical path 20 of light enteringthe GH cell 12 b, and can be moved out of the effective optical path 20by being shifted to the position shown by imaginary lines in FIG. 12.

FIG. 13A is a schematic plan view showing a specific example in whichthe polarizing plate 11 is secured to a moving part of a mechanical irisfor movement into and out of the effective optical path 20.

This mechanical iris is a mechanical diaphragm unit of the type which isgenerally used in digital still cameras, video cameras and the like, andis mainly made of two iris blades 18 and 19. The polarizing plate 11 isstuck to the iris blade 18.

As shown in FIG. 13B, as the iris blades 18 and 19 are moved upward anddownward by means of a driving motor which is not shown, the polarizingplate 11 moves upward and downward together with the iris blade 18. Byway of example, FIGS. 13B to 13D show on an enlarged state states whichtake place near the effective optical path 20 as the iris is graduallystopped down from its fully open state.

FIG. 13B shows the fully open state of the diaphragm, and in this state,the polarizing plate 11 secured to the iris blade 18 is also placed outof the effective optical path 20. As the iris blade 18 and the irisblade 19 are respectively moved upward and downward as shown by arrows21, the overlap of the iris blades 18 and 19 increases, and an aperture22 is narrowed as shown in FIG. 13C. At this time, the polarizing plate11 is moved into the effective optical path 20 and covers part of theaperture 22. Incidentally, FIG. 13A is a general view corresponding tothe state of FIG. 13C. FIG. 13D shows a state in which the iris isstopped down to a further extent, and in this state, the polarizingplate 11 covers the whole of the aperture 22.

Accordingly, according to the invention of the second prior application,in dark places, by shifting the polarizing plate 11 out of the effectiveoptical path 20 of light, it is possible to increase the maximumtransmittance to at least twice that of a device of the type in whichthe polarizing plate 11 is fixed, while in bright places, it is possibleto realize a light control operation of large optical density ratio bythe combination of the polarizing plate 11 and the GH cell 12 b.

Accordingly, according to each of the inventions of the first and secondprior applications, by constructing a light control device with a liquidcrystal element using a guest-host liquid crystal whose host material ismade of a negative liquid crystal and a polarizing plate disposed formovement into and out of the optical path of light entering this liquidcrystal element, it is possible to provide a light control device whichhas a large optical density ratio and can perform light controloperation over a wide range from bright places to dark places, as wellas an image pickup apparatus using the light control device.

Patent Document 1: Japanese Patent Application Publication No.2001-201769 (FIGS. 1 and 3)

Patent Document 2: Japanese Patent Application Publication No.H11-326894 (FIGS. 1 and 2)

However, in the case of this type of light control device using a GHcell, if high contrast ratio and high optical density ratio are to berealized, there is a need for a polarizing plate movable into and out ofan effective optical path, and there is also a need for a moving partfor moving the polarizing plate into and out the effective optical path.Accordingly, the light control device has the limitations of beingunable to be miniaturized with high contrast ratio realized.

In addition, the GH cell has problems such as the fact that its shadingperformance is not sufficient and the fact that manufacturing troubleseasily occur because of the use of an alignment film.

In view of the circumstances described above, an object of the presentinvention is to provide a light control device including a liquidcrystal element which does not need a polarizing plate and an alignmentlayer and is compact and high in both contrast ratio and optical densityratio, and further, can be driven at low applied voltage and can exhibitstable performance even if environmental temperature varies, and adriving method for the light control device, as well as an image pickupapparatus using the light control device.

DISCLOSURE OF THE INVENTION

The present inventor has conducted intensive research to solve theabove-mentioned problems, and has discovered that if polymer networkliquid crystal is used as a liquid crystal material, it is possible tosolve the problems by devising the method of use of the polymer networkliquid crystal.

Namely, the present invention relates to a light control device whichincludes a liquid crystal element having liquid crystal sealed betweenopposed substrates, the liquid crystal being polymer network liquidcrystal, the gap between the opposed substrates in an effective opticalpath being 4-11 μm.

The present invention also relates to a driving method for a lightcontrol device which includes a liquid crystal element having liquidcrystal sealed between opposed substrates, the liquid crystal beingpolymer network liquid crystal, the gap between the opposed substratesin an effective optical path being 4-11 μm. The driving method controlsan applied voltage for driving the liquid crystal element, according tothe environmental temperature of the liquid crystal element. The presentinvention further relates to an image pickup apparatus in which thelight control device is disposed in an optical path of its image pickupsystem.

According to the light control device of the present invention, sincethe polymer network liquid crystal is used as the liquid crystal to besealed into the liquid crystal element and light control operation isperformed by using the scattering of light by the polymer network liquidcrystal, there is no need for a polarizing plate nor an alignment film.Accordingly, it is possible to fabricate a compact light control device,and it is also possible to avoid troubles associated with the alignmenttreatment of liquid crystal. In addition, since the liquid crystalmolecules of the polymer network liquid crystal form a continuous layerin its three-dimensional network polymer, the polymer network liquidcrystal can be driven at low applied voltage.

In addition, the gap between the opposed substrates is made not lessthan 4 μm at which satisfactory shading performance can be obtainedduring the opaque state in which driving voltage is not applied, and notlarger than 11 μm at which satisfactory light transmittance can beachieved at the applied voltage of 3.3 V which is a practical sourcevoltage, whereby it is possible to realize a light control device whichhas high contrast ratio (optical density ratio) and can be driven at lowapplied voltage.

According to the driving method for the light control device of thepresent invention, the applied voltage for driving the liquid crystalelement is controlled according to the environmental temperature of theliquid crystal element, whereby even if the characteristics of theliquid crystal element vary according to the environmental temperature,the liquid crystal element can be made to offer stable light controlperformance.

In the image pickup apparatus of the present invention, since the lightcontrol device of the present invention is disposed in an optical pathof its image pickup system, it is possible to effectively use thefeatures of the light control device of the present invention.

Accordingly, the present invention is extremely useful in theminiaturization of light control devices using liquid crystal opticalelements and image pickup apparatuses having the light control devices,as well as in the improvement of the performance, image quality andreliability of the light control devices and the image pickupapparatuses.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1D are schematic perspective views and schematiccross-sectional views showing the principle of light control operationof a PN liquid crystal cell based on a mode for carrying out the presentinvention;

FIG. 2 is a graph showing one comparative example of the relationshipbetween the light transmittance of a PN liquid crystal cell and the cellgap thereof at varied applied voltages;

FIG. 3 is a graph showing one comparative example of the relationshipbetween the light transmittance and the applied voltage of a PN liquidcrystal cell at varied operating environmental temperatures of a liquidcrystal element;

FIG. 4A is a schematic cross-sectional view of a PN liquid crystal cell,while FIG. 4B is a schematic side view of a light control device usingthe PN liquid crystal cell;

FIG. 5 is a schematic cross-sectional view of a camera system in which alight control device is incorporated;

FIG. 6 is a block diagram including a driving circuit of a camerasystem;

FIG. 7 is a graph showing the relationship between the lighttransmittance and the applied voltage of the PN liquid crystal cellaccording to an embodiment of the present invention;

FIGS. 8A and 8B are schematic explanatory views showing the operatingprinciple of a related art light control device using a GH cell;

FIG. 9 is a graph showing the relationship between the lighttransmittance and the applied voltage of the related art light controldevice using a GH cell;

FIGS. 10A to 10C are schematic views showing the operating principle ofa light control device using a GH cell based on the invention of thefirst prior application;

FIG. 11 is a graph showing the relationship between the lighttransmittance and the applied voltage of a light control device using aGH cell;

FIG. 12 is a schematic side view of a light control device using a GHcell and a polarizing plate based on the invention of the second priorapplication; and

FIGS. 13A to 13D are a front view of a mechanical iris to which apolarizing plate is secured, and partial enlarged views showing lightcontrol operation occurring near an effective optical path of themechanical iris.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, optically transparent electrodes arepreferably provided on the opposed surfaces of the optically transparentopposed substrates, respectively. The gap between the opposed substratesis more preferably 6-10 μm.

The light control device of the present invention is preferably providedwith a temperature detecting section which detects the environmentaltemperature of the liquid crystal element, and a pulse control sectionwhich control the applied voltage for driving the liquid crystalelement, according to the environmental temperature detected by thetemperature detecting section, and the applied voltage is preferably aneffective AC pulse voltage.

A preferred embodiment of the present invention will be described belowwith reference to the accompanying drawings.

Optimization of Cell Gap of Polymer Network (PN) Liquid Crystal Cell

FIGS. 1A and 1B are schematic perspective views showing the principle oflight control operation of a polymer network (PN) liquid crystal cell 1,and FIGS. 1B and 1D are schematic cross-sectional views.

The PN liquid crystal cell 1 is fabricated by injecting a mixture inwhich a liquid crystal material, a polymer precursor such as a monomeror an oligomer, and a polymerization initiator are uniformly mixed intoan empty cell constituted by two transparent substrates 8 which arestuck together with a predetermined gap interposed therebetween and oneach of whose opposed surfaces a transparent electrode 9 (not shown ineither of the schematic perspective views) is formed, and thenperforming light irradiation or heating under appropriate conditions(such as the wavelength of irradiation light and the intensity,temperature and time of irradiation light) to polymerize the polymerprecursor.

In the uniform mixture, when a dense random three-dimensional networkpolymer (a polymer crosslinked in the form of a network) is formed bypolymerization, the generated network polymer 3 and liquid crystalmolecules 2 phase-separate to form polymer network liquid crystal havingthe structure that the polymer 3 is disposed in the continuous layer ofthe liquid crystal 2 in the form of a three-dimensional network.

When driving voltage is not being applied to the cell, as shown in FIG.1A, the liquid crystal molecules 2 are aligned along the random wallsurfaces of the network polymer 3 while forming a continuous layer inthe network polymer 3. As a result, the network polymer 3 forms lightscattering interfaces, and the incident light 5 is scattered at theinterfaces between the network polymer 3 and the liquid crystalmolecules 2.

On the other hand, when driving voltage is being applied to the cell, asshown in FIG. 1C, the liquid crystal molecules are oriented in apredetermined direction with respect to the electric field, so that thedirections of the liquid crystal molecules are aligned and therefractive index becomes constant in the traveling direction of light.Accordingly, the incident light 5 passes through without beingscattered.

As described above, the light control operation of the PN liquid crystalcell 1 is performed by using variations in the orientation state of theliquid crystal 2 charged into the gaps of the three-dimensional networkpolymer 3, whereby there is no need for a polarizing plate nor alignmenttreatment for liquid crystal molecules such as formation of an alignmentfilm.

In addition, the liquid crystal molecules 2 form the continuous layer inthe network polymer 3, whereby the liquid crystal molecules 2 can bedriven at low applied voltage.

FIG. 2 is a graph showing one comparative example of the relationshipbetween the light transmittance of a PN liquid crystal cell and the cellgap thereof (the gap between its transparent substrates) at variedapplied voltages (the environmental temperature of each liquid crystalelement is 25° C.). For example, the upper limit of a cell gap capableof achieving a light transmittance of 80% becomes larger at higherapplied voltages, and is approximately 4 μm at an applied voltage of 1V, slightly larger than 6 μm at an applied voltage of 2 V, slightly lessthan 9 μm at an applied voltage of 3 V, and approximately 10 μm at anapplied voltage of 3.3 V. Contrarily, in the case of a fixed cell gap,the light transmittance becomes larger at higher applied voltages; forexample, in the case of a cell gap of 10 μm, the light transmittance isslightly less than 15% at an applied voltage of 2 V and approximately75% at an applied voltage of 3 V, and reaches approximately 80% at anapplied voltage of 3.3 V, whereas in the case of a cell gap of 11 μm,the light transmittance is approximately 5% at an applied voltage of 2V, approximately 60% at an applied voltage of 3 V, and approximately72-73% at an applied voltage of 3.3 V, and reaches as low as 77-78% evenat an applied voltage of 4 V.

As described above, as the cell gap becomes larger, the driving voltagenecessary to achieve the maximum transmittance becomes higher. Forexample, if the transmittance necessary for light control operation isassumed to be over 70%, when the cell gap is larger than 11 μm, thedriving voltage necessary to light control operation exceeds 3.3 V andbecomes higher than the source voltages used in general consumerappliances, so that a booster circuit becomes necessary to supply thisdriving pulse. Accordingly, the cell gap is preferably not larger than11 μm so that a transmittance of over 70% can be achieved by using asource voltage of 3.3 V without boosting, more preferably not largerthan 10 μm so that a maximum cell transmittance of slightly larger than80% can be achieved.

On the other hand, in the case of an extremely small cell gap, when thecell is opaque with the driving voltage off, the proportion of lightwhich passes through the cell without being scattered therein increases,and the shading performance lowers. This corresponds to the graph of anapplied voltage of 0 V in FIG. 2. If the cell gap exceeds 6 μm, thelight transmittance of the cell which is opaque is negligibly small, butif the cell gap is smaller than 4 μm, the light transmittance exceeds3-4%, so that it is impossible to satisfactorily use the merit of PNliquid crystal cells capable of ensuring high contrast ratio with a cellalone without a polarizing plate in a compact manner. Accordingly, thecell gap is preferably not less than 4 μm so that satisfactory shadingperformance can be achieved, more preferably not less than 6 μm so thatmore complete shading performance can be achieved.

As described above, if an actual light control device effectively usingthe merit of a PN liquid crystal element is to be realized, the gap(cell gap) between its transparent substrates is controlled, preferablyto 4-11 μm, more preferably to 6-10 μm.

Temperature Dependence of Applied Voltage of Polymer Network (PN) LiquidCrystal Cell

FIG. 3 is a graph showing one comparative example of the relationship(so-called V-T characteristic) between the light transmittance of a PNliquid crystal cell and the applied voltage according to an embodimentwhich will be described later, at varied environmental temperatures ofthe liquid crystal element (the cell gap in this case is 10.7±0.1 μm.)It can be seen that as the environmental temperature decreases from 40°C. to 25° C. to −5° C., the V-T characteristic shifts toward the rightand higher applied voltages become necessary.

Unlike GH cells, PN liquid crystal cells do not need liquid crystalalignment films and their transient response times are not greatlyinfluenced by their cell gaps. However, as shown in FIG. 3, their V-Tcharacteristics are very strongly influenced by environmentaltemperature. This is considered to be because when the temperature ishigh, liquid crystal molecules exhibit active thermal motion, and hence,good response to electric fields, so that small applied voltages aresufficient, but as the temperature becomes lower, the motion of theliquid crystal molecules become slower and their responses to electricfields become lower, so that higher applied voltages become necessary toachieve the same light transmittance.

Accordingly, in consideration of this temperature characteristic, byconstantly monitoring the environmental temperature of a PN liquidcrystal element and optimally controlling an effective applied voltagefor driving the liquid crystal cell, according to the detectedenvironmental temperature, it is possible to realize a light controldevice capable of offering stable light control performance even if theenvironmental temperature varies.

Light Control Device

FIG. 4A is a schematic cross-sectional view of the PN liquid crystalcell 1. In the PN liquid crystal cell 1, the two transparent substrates8 having inside opposed surfaces on which the two transparent electrodes9 are respectively formed are held to oppose each other with apredetermined cell gap interposed therebetween, and a polymer networkliquid crystal 4, which is made of a continuous layer of the liquidcrystal molecules 2 in which the three-dimensional network polymer 3 isformed, is formed in the gap between the two transparent substrates 8.

As the transparent substrates 8, glass substrates are generally used,but are not limitative, and plastic substrates and the like may also beused. As the transparent electrodes 9, transparent conductive layerssuch as ITO (Indium Tin Oxide), FTO (Fluorine-doped tin oxide: tin oxidedoped with fluorine) and ATO (Antimony-doped tin oxide: tin oxide dopedwith antimony) are formed by vapor deposition and the like.

The PN liquid crystal cell 1 is fabricated, for example, in thefollowing manner. First, the peripheral portion of the transparentsubstrates 8 on which the transparent electrodes 9 are respectivelyformed in advance is coated to a predetermined width with a sealing(seal) material 10 made of a thermosetting epoxy resin. This sealingmaterial 10 contains, as spacers, a glass fiber having the same diameteras the cell gap. Then, the two transparent substrates 8 are aligned andsuperposed on each other, and are then subjected to heat treatment inthe state of being pressed under appropriate conditions (for example,150-170° C., 1-2 kg/cm²) by means of a heat press plate or the like,thereby curing the sealing material 10 in the peripheral portion tofabricate an empty cell in which liquid crystal is not yet sealed.

Then, the interior of the empty cell is evacuated to vacuum, and amixture in which a liquid crystal material, a polymer material such aspolymer precursor, and a polymerization initiator are uniformly mixed isinjected into the interior of the empty cell. After that, this mixtureis irradiated with ultraviolet rays under appropriate conditions (forexample, 15-100 mW/cm², 25-50° C., 30-120 seconds) to polymerizemonomers and form the polymer network liquid crystal 4, therebyfinishing the PN liquid crystal cell 1.

The liquid crystal material may be of any kind that is generallyaccepted as a liquid crystal material in this technical field, and isnot limited to a single liquid crystal compound and may also be amixture which contains two or more kinds of liquid crystal compounds aswell as substances other than liquid crystal compounds.

The liquid crystal material preferably has positive dielectric constantanisotropy. In addition, nematic liquid crystal, smectic liquid crystaland cholesteric liquid crystal are preferable, and uniaxial nematicliquid crystal is particularly preferable. The liquid crystal materialmay further appropriately contain an substance for improving itsperformance.

The polymer precursor may be, for example, a monomer and/or an oligomerprepared by radical addition polymerization, a monomer and/or anoligomer prepared by cation addition polymerization, and a monomerand/or an oligomer prepared by ring-opening polymerization. Athree-dimensional network polymer having gaps of desired size may alsobe formed in advance so that further polymerization is performed byusing this network polymer as a material.

Specific examples of the liquid crystal material and the polymermaterial are described in Japanese Patent Application Publication No.2000-66173 and the like.

FIG. 4B is a schematic side view of a light control device using the PNliquid crystal cell 1. A light control device 23 is basically made ofonly the PN liquid crystal cell 1. If necessary, an iris unit such as amechanical iris may be added. The PN liquid crystal cell 1 is disposedbetween a front lens group 15 and a rear lens group 16 each constructedof a plurality of lenses, for example, like a zoom lens. Light passingthrough the front lens group 15 enters the PN liquid crystal cell 1, andthe light passing through the PN liquid crystal cell 1 is converged bythe rear lens group 16 and formed on an image pickup plane 17 as animage.

Image Pickup Apparatus

FIG. 5 shows an example of a CCD (Charge coupled device) camera in whichthe light control device 23 according to the present embodiment isincorporated.

Namely, in a CCD camera 50, a first lens group 51 and a second lensgroup (for zooming) 52 which correspond to the front lens group 15, athird lens group 53 and a fourth lens group (for focusing) 54 whichcorrespond to the rear lens group 16, and a CCD package 55 are arrangedin that order at appropriate space intervals along an optical axis shownby a dot-dashed line, and an infrared cut-off filter 55 a, an opticallow-pass filter unit 55 b and a CCD image pickup element 55 c areincorporated in the CCD package 55.

The light control device 23 made of the PN liquid crystal cell 1 basedon the above-mentioned invention is secured on the same optical pathbetween the second lens group 52 and the third lens group 53 at aposition closer to the third lens group 53 for the purpose ofcontrolling the amount of light (reducing the amount of light). Thefourth lens group 54 for focusing is disposed for movement between thethird lens group 53 and the CCD package 55 along the optical path bymeans of a linear motor 57. The second lens group 52 for zooming isdisposed for movement between the first lens group 51 and the lightcontrol device 23 along the optical path.

Driving Circuit

FIG. 6 is a block diagram of the driving circuit of the above-mentionedCCD camera.

According to this drawing, a CCD image pickup element 55 c is disposedon the light exit side of the light control device 23, and a CCD drivingcircuit section 60 is connected to the CCD image pickup element 55 c.The output signal of the CCD image pickup element 55 c is processed in aY/C signal processing section 61, and a luminance signal (Y signal) isfed back to a control circuit section 62 of a PN liquid crystal celldriving control unit 64.

The PN liquid crystal cell driving control unit 64 which also generatesdriving pulses to be applied to the PN liquid crystal cell 1 is made ofthe control circuit section 62 and a pulse generating circuit section63, and generates driving pulses which are controlled in frequency,pulse voltage and pulse width. The environmental temperature of the PNliquid crystal cell 1 is detected by a thermistor 65, and this detectedtemperature information is inputted to the control circuit section 62.

The flow of a control signal is as follows. The luminance information (Ysignal) from the Y/C signal processing section 61 and the environmentaltemperature information of the PN liquid crystal cell 1 are fed back tothe control circuit section 62 of the PN liquid crystal cell drivingcontrol unit 64 together with base clocks outputted from the CCD drivingcircuit section 60. The control signal generated by the control circuitsection 62 in response to these signals is inputted to the pulsegenerating circuit section 63. Then, in the pulse generating circuitsection 63, AC pulses whose effective pulse voltage is optimallycontrolled according to the detected environmental temperature of the PNliquid crystal cell 1 are generated in synchronism with the base clocks,and are applied to the PN liquid crystal cell 1.

It is to noted that the light control device and the image pickupapparatus according to the present invention are suited to the casewhere driving electrodes for the liquid crystal element are formed overat least the entire area of an effective light transmission section. Bycontrolling driving pulses to be applied to the driving electrodesformed in this state, it is possible to perform total control on lighttransmittance with high accuracy over the entire effective optical pathwidth.

As described above, according to the light control device according tothe embodiment of the present invention, and the driving method for thesame, as well as the image pickup apparatus using the light controldevice, the cell gap of the PN liquid crystal cell using the polymernetwork liquid crystal is controlled to 4-11 μm and an effective voltageto be applied to the cell is optimized, whereby light control operationwhich is high in contrast ratio and optical density ratio can berealized in a compact form without using a polarizing plate and at lowapplied voltage. The light control device can stably offer itsperformance even if the environmental temperature varies. In addition,since an alignment film is not used, it is possible to avoid troublesassociated with the alignment treatment of liquid crystal.

Embodiment

A preferred embodiment of the present invention will be specificallydescribed below with reference to the accompanying drawings.

The PN liquid crystal cell 1 shown in the fabrication view 4A of the PNliquid crystal cell was fabricated in the following manner. First, theperipheral portion of the transparent substrates on which the patternsof the transparent electrodes 9 using ITO films were respectively formedin advance was coated to a predetermined width with the sealing (seal)material 10 made of a thermosetting epoxy resin. This sealing material10 was made to contain, as spacers, a glass fiber of diameter 10.8 μm.

Then, the two transparent substrates 8 were aligned and superposed oneach other, and were then subjected to heat treatment with 150-170° C.and 1-2 kg/cm² by means of a heat press plate, thereby curing thesealing material 10 in the peripheral portion to fabricate an empty cellin which liquid crystal was not yet sealed.

The cell gap of the obtained liquid crystal was measured with ameasuring device using the interference of light. The gap in the cellcenter was approximately 10.6 μm, and the gap in the cell periphery wasapproximately 10.8 μm.

The interior of this empty cell was evacuated to vacuum, and afterPNM-172 manufactured by DAINIPPON INK AND CHEMICALS, INCORPORATED (amixture of liquid crystal and monomers and the like) was injected intothe empty cell as a PN liquid crystal material, the monomers werepolymerized by irradiation with ultraviolet rays under the conditions of15-100 mW/cm², 25-50° C. and 30-120 seconds, whereby the polymer networkliquid crystal 4 was formed between the two transparent electrodes 9 andthe PN liquid crystal cell 1 was finished.

FIG. 7 is a graph showing the relationship (V-T characteristic) betweenthe light transmittance of the light control device using the finishedPN liquid crystal cell 1 and the voltage applied to the PN liquidcrystal cell 1. Variations in the light transmittance were measuredwhile driving pulses of rectangular waves were being applied. As theapplied voltage increased, the average visible light transmittanceincreased from a minimum transmittance of several % to over 80%.

Although this V-T characteristic differs according to liquid crystalcell structures or constituent materials to be used, the PN liquidcrystal cell 1 according to the present embodiment reached a maximumtransmittance of approximately 82% with respect to the application of apulse voltage of ±4 V or more (50 Hz).

FIG. 3 shows the V-T characteristic at each of the environmentaltemperatures of 40° C., 25° C. and −5° C. It can be seen that as theenvironmental temperature decreases, the V-T characteristic shiftstoward the right and higher applied voltages become necessary.Accordingly, in consideration of this temperature characteristic, byconstantly monitoring the environmental temperature of the PN liquidcrystal element and optimally controlling an effective driving pulsevoltage to be applied to the liquid crystal cell, according to thedetected environmental temperature, it is possible to realize the lightcontrol device capable of performing stable light control performanceeven if the environmental temperature varies.

Although the invention of the present application has been describedabove on the basis of the mode for carrying out the invention as well asthe embodiments, the present invention is not limited to any of theseexamples, and it goes without saying that it is possible to selectappropriate ones from among sample structures, materials to be used,driving methods for liquid crystal cells, the forms of light controldevices, driving mechanisms and the like, without departing from thegist of the invention.

For example, in the present embodiment, reference has been made to anexample using pulse voltage modulation (PHM) as a driving method for aliquid crystal cell, but the present embodiment can also be applied tothe case in which the liquid crystal cell is driven by pulse widthmodulation (PWM).

In addition, the light control device according to the present inventioncan be widely applied to not only optical diaphragm units for imagepickup apparatuses such as the above-mentioned CCD camera, but alsovarious optical systems for light amount control for electrophotographiccopying machines, optical communication equipment and the like.

In addition, the light control device according to the present inventioncan of course be applied to not only image pickup devices such as CCDs(Charge Coupled Devices) of the type used in the present embodiment, butalso CMOS (Complementary Metal-Oxide Semiconductor) image sensors andthe like.

Furthermore, the light control device according to the present inventioncan be applied to not only optical filters but also various imagedisplay elements for displaying characters or images.

Furthermore, the structure and the material of the above-mentionedliquid crystal optical element as well as the constructions and the likeof the driving mechanism, the driving circuit and the control circuit ofthe liquid crystal optical element can be variously modified. From amongrectangular waves, trapezoidal waves, triangular waves and sine waves,any driving waveform can be used for driving, whereby the orientation ofliquid crystal is varied according to the potential difference betweentwo electrodes which constitute the liquid crystal cell, to control thelight transmittance thereof. In addition, means for measuring theenvironmental temperature of the liquid crystal element is not limitedto a thermistor, and may also use other temperature sensors.

INDUSTRIAL APPLICABILITY

According to the light control device of the present invention, sincepolymer network liquid crystal is used as liquid crystal to be sealedinto its liquid crystal element and light control operation is performedby using the scattering of light by the polymer network liquid crystal,there is no need for a polarizing plate nor an alignment film.Accordingly, it is possible to fabricate a compact light control device,and it is also possible to avoid troubles associated with the alignmenttreatment of liquid crystal. In addition, since the liquid crystalmolecules of the polymer network liquid crystal form a continuous layerin its three-dimensional network polymer, the polymer network liquidcrystal can be driven at low applied voltage.

In addition, the gap between the opposed substrates is made not lessthan 4 μm at which satisfactory shading performance can be obtainedduring the opaque state in which driving voltage is not applied, and notlarger than 11 μm at which satisfactory light transmittance can beachieved at the applied voltage of 3.3 V which is a practical sourcevoltage, whereby it is possible to realize a light control device whichhas high contrast ratio (optical density ratio) and can be driven at lowapplied voltage.

According to the driving method for the light control device of thepresent invention, the applied voltage for driving the liquid crystalelement is controlled according to the environmental temperature of theliquid crystal element, whereby even if the characteristics of theliquid crystal element vary according to the environmental temperature,the liquid crystal element can be made to offer stable light controlperformance.

In the image pickup apparatus of the present invention, since the lightcontrol device of the present invention is disposed in an optical pathof its image pickup system, it is possible to effectively use thefeatures of the light control device of the present invention.

Accordingly, the present invention is extremely useful in theminiaturization of light control devices using liquid crystal opticalelements and image pickup apparatuses having the light control devices,as well as in the improvement of the performance, image quality andreliability of the light control devices and the image pickupapparatuses.

1. A light control device comprising: opposing substrates with a gaptherebetween; liquid crystal in said gap sealed between said opposingsubstrates, said liquid crystal being a polymer network liquid crystal;optically transparent electrodes on gap-side surfaces of each of saidopposing substrates and in contact with said liquid crystal; an imageprocessing unit which detects a luminance signal of said liquid crystal;a temperature detecting unit which detects a temperature of said liquidcrystal; and a pulse control unit which controls a width of a pulse ofan applied voltage for driving said liquid crystal, the pulse controlunit controlling said width of said pulse according to both of saidtemperature detected by said temperature detecting unit and saidluminance detected by said image processing unit, wherein, said gapbetween said opposing substrates along an effective optical path has awidth between about 4 μm and about 11 μm.
 2. The light control deviceaccording to claim 1, wherein said gap width is between about 6 and 10μm.
 3. The light control device according to claim 1, wherein saidopposing substrates are optically transparent.
 4. The light controldevice according to claim 1, wherein said applied voltage is an AC pulsevoltage.
 5. A method for driving a light control device having opposingsubstrates with a gap therebetween, liquid crystal in said gap sealedbetween said opposing substrates, said liquid crystal being a polymernetwork liquid crystal, optically transparent electrodes on gap-sidesurfaces of each of said opposing substrates and in contact with saidliquid crystal, an image processing unit which detects a luminancesignal of said liquid crystal, a temperature detecting unit whichdetects a temperature of said liquid crystal, a pulse control unit whichcontrols a width of a pulse of an applied voltage for driving saidliquid crystal based on both of said temperature detected by saidtemperature detecting unit and said luminance signal detected by saidimage processing unit, and said gap between said opposing substratesalong an effective optical path has a width between about 4 μm and about11 μm, said driving method comprising: applying a voltage for drivingsaid liquid crystal element; detecting a temperature of said liquidcrystal element; and controlling said applied voltage for driving saidliquid crystal element, according to the detected temperature of saidliquid crystal element.
 6. The method for driving a light control deviceaccording to claim 5, wherein said applied voltage is an AC pulsevoltage.
 7. An image pickup apparatus, wherein the light control deviceaccording to any of claims 1, 2, 3, or 4 is disposed in an optical pathof an image pickup system of said image pick up apparatus.
 8. The lightcontrol device according to claim 1, wherein said detected temperatureis an environmental temperature.